Picking GENNODES brings up the sub-menu:

which permits deletion or creation of nodes within the working polygon (the currently-activated polygon). It is advisable to use DISPLAY to make sure that the whole working polygon is in the display window before carrying out any operation affecting all the nodes in this polygon.

DELETE removes all interior nodes present in the active polygon. This is normally required before any new set of nodes is created by means of GENERATE.

Option DEPGRID permits input of depth information needed in the course of creating a new set of internal nodes within the working polygon. The user is prompted to enter the name of a file in NEIGH format containing a reference depth grid which covers at least the working polygon.

The next option, DEPMESH, lays out a regular Cartesian grid over the working polygon and computes depths at the centre of each grid square within the working polygon, by linear interpolation among the depths in the reference depth grid. Where the centre of a grid square lies within roughly half a mesh interval from the domain boundary, the depth is flagged negative, so that the square in question can be ruled out as a seed square in later node creation calculations. Grid squares outside the working polygon or land boundary are assigned a depth of zero and subsequently ignored. How individual grid squares are treated can be checked visually by enabling the option "Depth types" in the following right-hand panel, which is displayed when DEPMESH is invoked:

The user can change the quantities displayed in red in this panel. If required for diagnostic purposes, mesh data can be output in MESH format (see Appendix A) by Picking "NONE" and then supplying a file name for the output. The user has control over the mesh size for the Cartesian grid, which is basic to all three ways of generating new nodes offered under option GENERATE (see below). The default value of mesh size shown (in problem length units) is found by taking the greater of the lengths of the active polygon in the x- and y-directions and dividing this length by 50 to get a mesh size which gives a Cartesian grid covering the working polygon with 50 rows or columns. To set a different mesh size, Pick the displayed value with the cursor and enter a new value via the keyboard. Any of the three "Display Options" can be toggled by Picking the current status word. Further explanatory information comes up in the right-hand panel if either of the last two display options is toggled on. Turning on "Depth Triangles" displays triangles in the irregular depth grid which are actually used in evaluating depths in squares of the Cartesian grid. When the required options have been chosen, Pick ACCEPT to initiate the calculation of depth in each square of the Cartesian grid and subsequent display of other information specified.

Option GENERATE gives the user a choice of three different methods of generating a set of new interior nodes to fill the working polygon. The sub-menu providing these is:

Sub-option CLUSTERS:

The CLUSTERS option provides a method of creating a set of nodes whose spacing is a function of water depth. As explained in Henry (1988), when the model domain is subdivided into cells whose areas are proportional to water depth and the centres of area of these cells are taken as the basis for a triangular network, the areas of the triangles in the network are also approximately proportional to water depth and the spacing of the nodes is such that the Courant criterion is satisfied approximately throughout the grid, that is, node spacing is proportional to the local phase speed of shallow water waves. It should be noted that triangle area can be made proportional to any scalar quantity defined over the domain, by providing a reference grid for that quantity at stage DEPGRID above, in place of a depth grid.

In the NODER, the model domain is subdivided into cells by forming appropriately-sized compact clusters of squares belonging to the fine resolution Cartesian grid laid out over the domain using option DEPMESH. Cluster (cell) area is related to water depth by an expression A0 + A1*DEPTH + A2*DEPTH**2 , where the coefficients A0, A1, A2 can be set by the user. The linear case described in the preceding paragraph corresponds to A0 = A2 = 0.

Choice of option CLUSTERS brings up an information panel on the right, which allows the user to reset various parameters and display conditions. The information panel contents are:

Minimum cluster size (default =1) is reckoned in Cartesian grid squares. The maximum number of new nodes shown is the maximum node array size permitted minus the number of existing nodes in the whole domain. If a starting location for cluster formation is required other than at the point of maximum water depth, the starting location has to be specified in terms of number of rows and columns in the fine Cartesian grid, counting from the lower left-hand corner. The outlines of the clusters of squares from the Cartesian grid can be displayed as they are formed by Picking the option "Draw Clusters". Cluster formation begins on Picking option ACCEPT.

Sub-option UNIFORM:

Option UNIFORM places a new interior node at the centre of each square of the Cartesian grid whose centre lies within the active polygon and more than half a mesh interval from the nearest land boundary, that is, at the centre of each square assigned a positive depth as a result of operation DEPMESH above.

Sub-option RANDOM:

Option RANDOM places a new interior node at the centres of randomly chosen members of the set of eligible squares defined in the above discussion of the UNIFORM option. Node selection is controlled by parameters in the right-hand panel:

The seed number is an arbitrary number between 0 and 1 used in the pseudo-random number generator. It can be changed by Picking the existing value and entering another. The default value shown against "Number of Nodes Required" is the maximum number of nodes which can be generated by means of the RANDOM (or UNIFORM) node-generating option, i.e. the maximum number of allowable nodes in the current version of NODER minus the number of nodes that currently exist. To set the number of randomly placed nodes to be generated in the working polygon, Pick the default value and enter the number required.

The variable "Minimum Node Closeness" controls the minimum distance to be permitted between the new nodes, expressed as (number of Cartesian grid squares in x-direction + number of squares in y-direction). Node generation begins when AACEPT is Picked.

Sub-menu DISPLAY

This permits the same general control over windowing as in {TOP}DISPLAY described later. It is described in detail in Section